Motor vehicles, and in particular trucks, are a critical component in transporting materials, goods and people from place to place. The amount of energy required to move such vehicles depends on many factors. For instance, a substantial amount of energy is expended to overcome the resistance encountered in moving the vehicle through air. The amount of energy expended depends in large part on the aerodynamic drag force exerted on the vehicle by the air. In the field of surface transportation, and particularly in the long-haul trucking industry, even a small reduction in their aerodynamic drag results in improvements in fuel efficiency that can reduce annual operating costs significantly.
As generally known, a vehicle moving through air experiences a drag force, which may be divided into two components: frictional drag and pressure drag. Frictional drag comes from friction generated generally through the boundary layer as the vehicle passes through the air. Pressure drag results from the net pressure forces exerted as the air flows around the vehicle. The distinction between frictional drag and pressure drag is useful because the two types of drag are due to different flow phenomena. Frictional drag is typically most important for attached flows—that is, where the flow boundary layer has not separated from the vehicle surfaces, and is related to the surface area exposed to the flow. Pressure drag dominates for separated flows, and is generally related to the cross-sectional area of the vehicle facing the air stream. When the drag on vehicle is dominated by pressure drag forces, it will expend far more energy traveling through air than the same vehicle dominated by friction drag forces. It is therefore advantageous in the design of a vehicle to reduce pressure drag forces; thereby increasing the aerodynamic properties and efficiency of the vehicle.
A bluff body, such as a conventional truck hood or front section, produces significant pressure drag at typical highway speeds. One reason for the large pressure drag is the presence of a sharp angle located at a leading edge of the truck hood. More specifically, typical truck front sections include a substantially vertical front surface or grille that meets, along an upper edge, a generally horizontal hood surface. Referring to FIG. 1, a perspective view of a prior art Class 8 truck 10 showing an airstream 12 flowing over a hood 16 is depicted. The depicted air stream 12 encounters the conventionally shaped Class 8 truck 10 at the substantially vertical surface of the front surface or grille 14. It will be appreciated that for purposes of the present aerodynamic discussion, the truck's forward motion at highway speeds is equivalent to an air stream 12 having a similar but opposite velocity flowing over a stationary truck. The air stream passing over the front section, therefore, must negotiate an abrupt change in direction at the edge where the hood structure transitions from a substantially vertical orientation to a substantially horizontal orientation. As the air stream 12 turns upwardly as it negotiates the grille 14, it separates at a leading edge 16 of the hood 18, thereby forming a highly turbulent or wake region 22 of air located directly above the top surface of the hood and aft of the leading edge 16. Because of the presence of the large wake region 22 and pressure losses due to eddy formation in the wake region, drag increases on the vehicle.
Furthermore, in practical applications, the air stream 12 will include ubiquitous highway particulates, e.g. road grime, which are circulated in the eddies formed in the wake region 22. The eddy driven recirculation of the grime results in an increased rate of deposition of the particulates contained in the air stream 12 upon the hood 18 and windshield 20. This results in a high rate of road film build-up—thus impairing the driver's vision, and therefore safety, and increasing the amount of labor and stops required to keep the truck's windshield clear, resulting in inefficiency and increased costs.